Rotational restraint of flanges in I beam

[JSF10]

Dear all, just a quick question.....Basically, I have an I beam, and I want to find the shear buckling stress of the web. I want to model the top and bottom of the WEB with SPRINGS to take the rotational restraint of the FLANGES into account in the FEM program (Nastran). I am trying to calculate the K rotational restraint value to input into Nastran.

It will be in terms of a moment per unit angle of rotation per unit length???.

Any thoughts on this ?

Perhaps the easier way would be to model the flanges as beams connected to a shear web, and that's the way we normally do aircraft structure. But I was wondering if I could do with just springs and would prefer that.

Many thanks for looking into it.

 

[rb1957]

i'm not entirely sure this'll get you where you want to go. in part you'll need NL and large disp, but i think it is alot of complication without significant improvement in accuracy

in any case, springs connect grids, so the stiffness would need to reflect the grid spacing.

 

[JoshPlumSE]

AISC has some pretty good formulas (based on some pretty good research) about what the shear buckling stress is for the web of a wide flange.

I would think it would be a lot easier just to look up the research that has already be done rather than try to re-invent the wheel with an FEM analysis.

 

[GBor]

I've waited to see how others respond, but since response seems to be limited, I have a question and a comment:

Question: Why do you prefer to do it this way? Computer resources? easier to copy springs along a length and can place them strategically? I don't think I follow your desire.

Comment:

Perhaps the easier way would be to model the flanges as beams connected to a shear web, and that's the way we normally do aircraft structure.

Really? For global models, maybe (I don't model an entire aircraft), but for local models, I usually model any stringers (longerons) and frames (with shear ties) as plates. Then I discreetly model the fasteners using beams and the Huth fastener values. I've seen this approach from several other analysts as well.



Garland E. Borowski, PE
Engineering Manager
Star Aviation